The present invention relates to bearings which are used in the fields requiring high cleanness of a working area, such as food processing industry, semiconductor manufacturing industry, or the like, and therefore are fed with no lubricant.
Bearings for which such lubricants as lubricating oil and grease can not be used require the use of solid lubricants in order to reduce the friction between inner and outer races and rolling elements.
Conventionally, a bearing as shown in FIG. 9 has been known as an example of such bearings. The bearing comprises an inner ring 1, an outer ring 2, a plurality of steel balls 3 (only one is shown), and a cage 5 for retaining the steel balls 3. The inner and outer rings 1 and 2 are formed of a metal having no self-lubricativity, and the cage 5 of a fluoroplastic having self-lubricativity. During the revolutions of the bearing, the fluoroplastic is transferred to the steel ball, and then the inner and outer rings 1 and 2 in sequence to form a fluoroplastic film over them, whereby a lubricating action being performed.
The bearing of such a combination of materials, however, has a drawback that the lubricating action between the steel balls 3 and the metal steel inner and outer rings 1 and 2, can not be well performed at the beginnings of revolutions since it takes time for the fluoroplastic of the cage 5 to transfer to the surface of the steel balls 3.
In addition to the above conventional example, another type of a bearing is known, as disclosed in the Japanese Patent Application Laid-open No. 51-110152. This bearing comprises inner and outer rings, a cage, balls all of which are made of the same material. This material is a sintered carbonaceous material made from the mixture of black lead, carbon black, coke, and the like of a appropriate mixture ratio, including, if necessary, carbon fibers, so that it has a self-lubricativity. Accordingly it is assured that the bearing of this type obtains high lubricating properties.
Hardness of the sintered carbonaceous material, however, is so low as shown by values 30 to 120 of Shore hardness (corresponding to 207 of Vickers hardness), and therefore, the bearing made of this material can not bear a high load.